The present invention relates to a process for forming a phosphate coating having excellent paint adhesion and corrosion resistance on the surface of zinc or an alloy thereof without generating any sludge or waste water.
Processes for forming a phosphate coating on zinc or an alloy thereof generally comprise (1) a surface conditioning step, e.g., with colloidal titanium phosphate, (2) chemical conversion step with a phosphating solution, (3) rinsing step with water, (4) post-treatment step with a chromate solution and (5) drying step. When the zinc surface has been pretreated to provide a temporary chromate coating, the surface chromate coating is removed prior to the above-mentioned 5 steps by polishing the surface as by means of a wet buff wheel. The diluted waste of the phosphating solution from rinsing step (3) and the autodraining or periodic dumping of the chromate solution cause pollution problems. In addition, sludge formed during the chemical conversion process creates an eventual solid waste treatment or disposal problem.
A main component of sludge formed as a by-product of phosphate coating comprises Zn.sub.3 (PO.sub.4).sub.2.4H.sub.2 O which is the same as the main component of the phosphate coating. The formation of sludge results in part from the fact that the amount of dissolved zinc from the articles treated exceeds the amount of zinc in the deposited coating. It is also believed that the presence of nitrate catalyzes the removal of zinc from the surface and therefore increases the rate of sludge formation because of complex formation of NH.sub.3 (reduced NO.sub.3.sup.- with zinc ion.
In conventional phosphating solutions, when the amount of zinc dissolved by the above-mentioned reaction mechanism exceeds that of the zinc for forming the coating, sludge is formed.
The inventors have found that the formation of a phosphate coating can proceed relatively rapidly by using hydrogen peroxide in place of NO.sub.3.sup.- as an oxidizing agent in the phosphating solution for zinc but the resulting coating is disadvantageous because of poor adhesion of the phosphate coating to the substrate and poor adhesion of subsequently applied paint.